Add two-way messaging view

> Once an inbound message has been received, there should be a way to
> see the other messages in the system from the same service to the same
> number. Both in and outbound. Nice inbox/whatsapp stylee view or some
> such. This way the context of the reply is understood.
>
> Initially will only see the outbound template, not the actual message,
> but we’re going to change this for the rest (soon), so that you can
> always see the full message for all outbound.

app/main/views/conversation.py

@@ -0,0 +1,55 @@
+from flask import (
+    render_template,
+    url_for,
+)
+from flask_login import login_required
+from notifications_utils.recipients import format_phone_number_human_readable
+from notifications_utils.template import SMSPreviewTemplate
+from app.main import main
+from app.utils import user_has_permissions
+from app import notification_api_client, service_api_client
+
+
+@main.route("/services/<service_id>/conversation/<notification_id>")
+@login_required
+@user_has_permissions('view_activity', admin_override=True)
+def conversation(service_id, notification_id):
+
+    user_number = get_user_number(service_id, notification_id)
+
+    return render_template(
+        'views/conversations/conversation.html',
+        conversation=get_sms_thread(service_id, user_number=user_number),
+        user_number=user_number,
+    )
+
+
+def get_user_number(service_id, notification_id):
+    try:
+        user_number = service_api_client.get_inbound_sms_by_id(service_id, notification_id)['user_number']
+    except Exception:
+        user_number = notification_api_client.get_notification(service_id, notification_id)['to']
+    return format_phone_number_human_readable(user_number)
+
+
+def get_sms_thread(service_id, user_number):
+    return [
+        {
+            'inbound': ('notify_number' in notification),
+            'content': SMSPreviewTemplate(
+                {
+                    'content': notification.get('content') or notification['template']['content']
+                }
+            ),
+            'created_at': notification['created_at'],
+            'status': notification.get('status'),
+            'id': notification['id'],
+        }
+        for notification in sorted(
+            (
+                notification_api_client.get_notifications_for_service(service_id, to=user_number)['notifications'] +
+                service_api_client.get_inbound_sms(service_id, user_number=user_number)
+            ),
+            key=lambda notification: notification['created_at'],
+        )
+    ]